Notes of Ch 7 Landforms and their

Evolution| Class 11th Geography

 29 May, 2020

Notes of Ch 7 Landforms and their Evolution| Class 11th Geography

Introduction

• Landform: In simple words, small to medium tracts or parcels of the earth’s surface
are called landforms.

• Each landform has its own physical shape, size, materials and is a result of the action
of certain geomorphic processes and agent

• Several landforms together are called landscape. Each landform has its own shape,
size and materials Geomorphological processes are slow but significant in long run.

• Every landform has a beginning, they change their shape and composition in course
of time.

• Due to changes in climate and vertical and horizontal movements landforms change
their shape.

• Each landform undergo three stages called youth, mature and old stages.

• Geomorphology is the science of landforms Various geomorphic agents bring the

changes to the landforms such as running water, moving ice, wind glaciers,
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 underground water, waves by erosion and deposition.

• Each geomorphological agent produces its own assemblage of landforms. Most of

the geomorphological processes are imperceptible. The study of the landforms reveals
that the stage structure and process of land forms. They produce erosional and
depositional features.

Factors influencing erosion

• Rock structures such as fold, fault, joints, fractures, hardness, softness permeability,
and
impermeability.

• Stability of sea level

• Tectonic stability of landmass

• Climate

Running Water

• In humid regions, which receive heavy rainfall running water is considered the most
important of the geomorphic agents in bringing about the degradation of the land
surface.

• There are two components of running water.

→ One is overland flow on general land surface as a sheet.
→ Another is linear flow as streams and rivers in valleys.

• Most of the erosional landforms made by running water are associated with
vigorous and youthful rivers flowing over steep gradients.

• With time, stream channels over steep gradients turn gentler due to continued
erosion, and as a consequence, lose their velocity, facilitating active deposition.

• The gentler the river channels in gradient or slope, the greater is the deposition.
When the stream beds turn gentler due to continued erosion, downward cutting
becomes less dominant and lateral erosion of banks increases and as a consequence

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 the hills and valleys are reduced to plains.

Overland flow

• It causes sheet erosion.

• Depending upon irregularities of the land surface, the overland flow may
concentrate into narrow to wide paths.

• Because of the sheer friction of the column of flowing water, minor or major
quantities of materials from the surface of the land are removed in the direction of
flow and gradually small and narrow rills will form.

• These rills will gradually develop into long and wide gullies; the gullies will further
deepen, widen, lengthen and unite to give rise to a network of valleys.

• The divides between drainage basins are likewise lowered until they are almost
completely flattened leaving finally, a lowland of faint relief with some low resistant
remnants called monadnocks standing out here and there.

• This type of plain forming as a result of stream erosion is called a peneplain (an
almost plain.)

Youth

• Streams are few during this stage with poor integration and flow over original slopes
showing shallow V-shaped valleys with no floodplains or with very narrow floodplains
along trunk streams.

• Streams divides are broad and flat with marshes, swamp and lakes.

• Meanders if present develop over these broad upland surfaces. These meanders may
eventually entrench themselves into the uplands.

• Waterfalls and rapids may exist where local hard rock bodies are exposed.

Mature

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 • During this stage streams are plenty with good integration.

• The valleys are still V-shaped but deep; trunk streams are broad enough to have
wider floodplains within which streams may flow in meanders confined within the
valley.

• The flat and broad inter stream areas and swamps and marshes of youth disappear
and the stream divides turn sharp. Waterfalls and rapids disappear.

Old

• Smaller tributaries during old age are few with gentle gradients.

• Streams meander freely over vast floodplains showing natural levees, oxbow lakes,
etc.

• Divides are broad and flat with lakes, swamps and marshes. Most of the landscape is
at or slightly above sea level.

Erosional Landforms

Valleys

• Valleys start as small and narrow rills; the rills will gradually develop into long and
wide gullies; the gullies will further deepen, widen and lengthen to give rise to valleys.
Depending upon dimensions and shape, many types of valleys like V-shaped valley,
gorge, canyon, etc. can be recognised.

• A gorge is a deep valley with very steep to straight sides and a canyon is
characterised by steep step-like side slopes and may be as deep as a gorge. A gorge is
almost equal in width at its top as well as its bottom. In contrast, a canyonis wider at
its top than at its bottom. In fact, a canyon is a variant of gorge.

• Valley types depend upon the type and structure of rocks in which they form. For
example, canyons commonly form in horizontal bedded sedimentary rocks and gorges
form in hard rocks.

Potholes and Plunge Pools

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 • Over the rocky beds of hill-streams more or less circular depressions called potholes
form because of stream erosion aided by the abrasion of rock fragments.

• Once a small and shallow depression forms, pebbles and boulders get collected in
those depressions and get rotated by flowing water and consequently the depressions
grow in dimensions.

• A series of such depressions eventually join and the stream valley gets deepened. At
the foot of waterfalls also, large potholes, quite deep and wide, form because of the
sheer impact of water and rotation of boulders. Such large and deep holes at the base
of waterfalls are called plunge pools.

• These pools also help in the deepening of valleys. Waterfalls are also transitory like
any other landform and will recede gradually and bring the floor of the valley above
waterfalls to the level below.

Incised or Entrenched Meanders

• In streams that flow rapidly over steep gradients, normally erosion is concentrated
on the bottom of the stream channel.

• Also, in the case of steep gradient streams, lateral erosion on the sides of the valleys
is not much when compared to the streams flowing on low and gentle slopes.

• Because of active lateral erosion, streams flowing over gentle slopes, develop
sinuous or meandering courses.

• It is common to find meandering courses over floodplains and delta plains where
stream gradients are very gentle.

• But very deep and wide meanders can also be found cut in hard rocks. Such
meanders are called incised or entrenched meanders.

• Meander loops develop over original gentle surfaces in the initial stages of
development of streams and the same loops get entrenched into the rocks normally
due to erosion or slow, continued uplift of the land over which they start.

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 • They widen and deepen over time and can be found as deep gorges and canyons in
hard rock areas. They give an indication on the status of original land surfaces over
which streams have developed.

River Terraces

• River terraces are surfaces marking old valley floor or floodplain levels.

• They may be bedrock surfaces without any alluvial cover or alluvial terraces
consisting of stream deposits.

• River terraces are basically products of erosion as they result due to vertical erosion
by the stream into its own depositional floodplain.

• The river terraces may occur at the same elevation on either side of the rivers in
which case they are called paired terraces.

• When a terrace is present only on one side of the stream and with none on the other
side or one at quite a different elevation on the other side, the terraces are called
unpaired terraces.

• Unpaired terraces are typical in areas of slow uplift of land or where the water
column changes are not uniform along both the banks.

• The terraces may result due to:

(i) receding water after a peak flow
(ii) change in hydrological regime due to climatic changes
(iii) tectonic uplift of land
(iv) sea level changes in case of rivers closer to the sea.

Depositional Landforms

Alluvial Fans

• Alluvial fans are formed when streams flowing from higher levels break into foot
slope plains of low gradient.

• Normally very coarse load is carried by streams flowing over mountain slopes. This

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 load becomes too heavy for the streams to be carried over gentler gradients and gets
dumped and spread as a broad low to high cone shaped deposit called alluvial fan.

• Usually, the streams which flow over fans are not confined to their original channels
for long and shift their position across the fan forming many channels called
distributaries.

• Alluvial fans in humid areas show normally low cones with gentle slope from head to
toe and they appear as high cones with steep slope in arid and semi-arid climates

Deltas

• Deltas are like alluvial fans but develop at a different location.

• The load carried by the rivers is dumped and spread into the sea. If this load is not
carried away far into the sea or distributed along the coast, it spreads and
accumulates as a low cone.

• Unlike in alluvial fans, the deposits making up deltas are very well sorted with clear
stratification.

• The coarsest materials settle out first and the finer fractions like silts and clays are
carried out into the sea. As the delta grows, the river distributaries continue to
increase in length and delta continues to build up into the sea.

Floodplains, Natural Levees and Point Bars

• Deposition develops a floodplain just as erosion makes valleys.

• Floodplain is a major landform of river deposition. Large sized materials are

deposited first when stream channel breaks into a gentle slope. Thus, normally, fine
sized materials like sand, silt and clay are carried by relatively slow moving waters in
gentler channels usually found in the plains and deposited over the bed and when the
waters spill over the banks during flooding above the bed.

• A river bed made of river deposits is the active floodplain. The floodplain above the

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 bank is inactive floodplain.

• Inactive floodplain above the banks basically contain two types of deposits — flood
deposits and channel deposits.

• The flood deposits of spilled waters carry relatively finer materials like silt and clay.

• The flood plains in a delta are called delta plains.

• Natural levees are found along the banks of large rivers. They are low, linear and
parallel ridges of coarse deposits along the banks of rivers, quite often cut into
individual mounds.

• During flooding as the water spills over the bank, the velocity of the water comes
down and large sized and high specific gravity materials get dumped in the immediate
vicinity of the bank as ridges. They are high nearer the banks and slope gently away
from the river.

• The levee deposits are coarser than the deposits spread by flood waters away from
the river. When rivers shift laterally, a series of natural levees can form.

• Point bars are also known as meander bars.-They are found on the convex side of
meanders of large rivers and are sediments deposited in a linear fashion by flowing
waters along the bank.
almost uniform in profile and in width and contain mixed sizes of sediments.

• If there more than one ridge, narrow and elongated depressions are found in
between the point bars.
As the rivers build the point bars on the convex side, the bank on the concave side will
erode actively.

Meanders

• In large flood and delta plains, rivers rarely flow in straight courses. Loop-like channel
patterns called meanders develop over flood and delta plains.

• Meander is not a landform but is only a type of channel pattern. This is because of
propensity of water flowing over very gentle gradients to work laterally on the banks;

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 unconsolidated nature of alluvial deposits making up the banks with many
irregularities which can be used by water exerting pressure laterally; coriolis force
acting on the fluid water deflecting it like it deflects the wind.

• When the gradient of the channel becomes extremely low, water flows leisurely and
starts working laterally. Slight irregularities along the banks slowly get transformed
into a small curvature in the banks; the curvature deepens due to deposition on the
inside of the curve and erosion along the bank on the outside.

• Normally, in meanders of large rivers, there is active deposition along the convex
bank and undercutting along the concave bank.

• The concave bank is known as cut-off bank which shows up as a steep scarp and the
convex bank presents a long, gentle profile and is known as slip-off bank.

• As meanders grow into deep loops, the same may get cut-off due to erosion at the
inflection points and are left as ox-bow lakes

Braided Channels

• When rivers carry coarse material, there can be selective deposition of coarser
materials causing formation of a central bar which diverts the flow towards the banks;
and this flow increases lateral erosion on the banks.

• As the valley widens, the water column is reduced and more and more materials get
deposited as islands and lateral bars developing a number of separate channels of
water flow.

• Deposition and lateral erosion of banks are essential for the formation of braided
pattern.

Groundwater

• The surface water percolates well when the rocks are permeable, thinly bedded and
highly jointed and cracked.

• After vertically going down to some depth, the water under the ground flows
horizontally through the bedding planes, joints or through the materials themselves.

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 • It is this downward and horizontal movement of water which causes the rocks to
erode.

• Physical or mechanical removal of materials by moving groundwater is insignificant

in developing landforms. That is why, the results of the work of groundwater cannot
be seen in all types of rocks.
rocks like limestones or dolomites rich in calcium carbonate, the surface water as well
as groundwater through the chemical process of solution and precipitation deposition
develop varieties of landforms.

• Any limestone or dolomitic region showing typical landforms produced by the action
of groundwater through the processes of solution and deposition is called Karst
topography after the typical topography developed in limestone rocks of Karst region
in the Balkans adjacent to Adriatic sea.

• The karst topography is also characterised by erosional and depositional landforms.

Erosional Landforms

• Pools, Sinkholes, Lapies and Limestone Pavements.

• Small to medium sized round to sub-rounded shallow depressions called swallow

holes form on the surface of limestones through solution.

• Sinkholes are very common in limestone/karst areas. A sinkhole is an opening more

or less circular at the top and funnel-shapped towards the bottom with sizes varying
in area from a few sq. m to a hectare and with depth from a less than half a metre to
thirty metres or more.

• If the bottom of a sinkhole forms the roof of a void or cave underground, it might
collapse leaving a large hole opening into a cave or a void below (collapse sinks). The
term doline is sometimes used to refer the collapse sinks.

• When sink holes and dolines join together because of slumping of materials along
their margins or due to roof collapse of caves, long, narrow to wide trenches called
valley sinks or Uvalas form.

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 Caves

• In areas where there are alternating beds of rocks (shales, sandstones, quartzites)
with limestones or dolomites in between or in areas where limestones are dense,
massive and occurring as thick beds, cave formation is prominent.

• Water percolates down either through the materials or through cracks and joints
and moves horizontally along bedding planes.

• It is along these bedding planes that the limestone dissolves and long and narrow to
wide gaps called caves result.

• There can be a maze of caves at different elevations depending upon the limestone
beds and intervening rocks.

• Caves normally have an opening through which cave streams are discharged. Caves
having openings at both the ends are called tunnels.

Depositional Landforms

• Many depositional forms develop within the limestone caves. The chief chemical in
limestone is calcium carbonate which is easily soluble in carbonated water (carbon
dioxide absorbed rainwater).

• This calcium carbonate is deposited when the water carrying it in solution

evaporates or loses its carbon dioxide as it trickles over rough rock surfaces.

Stalactites, Stalagmites and Pillars

• Stalactites hang as icicles of different diameters. Normally they are broad at their
bases and taper towards the free ends showing up in a variety of forms.
Stalagmites rise up from the floor of the caves. In fact, stalagmites form due to
dripping water from the surface or through the thin pipe, of the stalactite,
immediately below it
Stalagmites may take the shape of a column, a disc, with either a smooth, rounded
bulging end or a miniature crater like depression.
The stalagmite and stalactites eventually fuse to give rise to columns and pillars of
different diameters.

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 Glaciers

• Masses of ice moving as sheets over the land (continental glacier or piedmont glacier
if a vast sheet of ice is spread over the plains at the foot of mountains) or as linear
flows down the slopes of mountains in broad trough-like valleys (mountain and valley
glaciers) are called glaciers.

• The movement of glaciers is slow. Glaciers move basically because of the force of
gravity.

• Erosion by glaciers is tremendous because of friction caused by sheer weight of the

ice.

• The material plucked from the land by glaciers (usually large-sized angular blocks
and fragments) get dragged along the floors or sides of the valleys and cause great
damage through abrasion and plucking.

• Glaciers can cause significant damage to even un-weathered rocks and can reduce
high mountains into low hills and plains.

Erosional Landforms

Cirque

• Cirques are the most common of landforms in glaciated mountains. The cirques
quite often are found at the heads of glacial valleys.

• The accumulated ice cuts these cirques while moving down the mountain tops. They
are deep, long and wide troughs or basins with very steep concave to vertically
dropping high walls at its head as well as sides.

• A lake of water can be seen quite often within the cirques after the glacier
disappears.

• Such lakes are called cirque or tarn lakes. There can be two or more cirques one
leading into another down below in a stepped sequence.

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 Horns and Serrated Ridges

• Horns form through head ward erosion of the cirque walls.

• If three or more radiating glaciers cut headward until their cirques meet, high, sharp
pointed and steep sided peaks called horns form.

• The divides between cirque side walls or head walls get narrow because of
progressive erosion and turn into serrated or saw-toothed ridges sometimes referred
to as arêtes with very sharp crest and a zig-zag outline.

• The highest peak in the Alps, Matterhorn and the highest peak in the Himalayas,
Everest are in fact horns formed through headward erosion of radiating cirques.

Glacial Valleys/Troughs

• Glaciated valleys are trough-like and U-shaped with broad floors and relatively
smooth, and steep sides.

• The valleys may contain littered debris or debris shaped as moraines with swampy
appearance.

• There can be hanging valleys at an elevation on one or both sides of the main glacial
valley. The faces of divides or spurs of such hanging valleys opening into main glacial
valleys are quite often truncated to give them an appearance like triangular facets.

• Very deep glacial troughs filled with sea water and making up shorelines (in high
latitudes) are called fjords/fiords.

Depositional Landforms

• The unassorted coarse and fine debris dropped by the melting glaciers is called
glacial till.

• Most of the rock fragments in till are angular to sub- angular in form. Streams form
by melting ice at the bottom, sides or lower ends of glaciers.

• Some amount of rock debris small enough to be carried by such melt-water streams

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 is washed down and deposited. Such glacio- fluvial deposits are called outwash
deposits.

• Unlike till deposits, the outwash deposits are roughly stratified and assorted. The
rock fragments in outwash deposits are somewhat rounded at their edges.

Moraines

• These are are long ridges of deposits of glacial till.

• Terminal moraines are long ridges of debris deposited at the end (toe) of the
glaciers.

• Lateral moraines form along the sides parallel to the glacial valleys. The lateral
moraines may join a terminal moraine forming a horse-shoe shaped ridge. There can
be many lateral moraines on either side in a glacial valley.

• Many valley glaciers retreating rapidly leave an irregular sheet of till over their valley
floors. Such deposits varying greatly in thickness and in surface topography are called
ground moraines.

• The moraine in the centre of the glacial valley flanked by lateral moraines is called
medial moraine.

Eskers

• When glaciers melt in summer, the water flows on the surface of the ice or seeps
down along the margins or even moves through holes in the ice.

• These waters accumulate beneath the glacier and flow like streams in a channel
beneath the ice.

• Such streams flow over the ground (not in a valley cut in the ground) with ice
forming its banks.

• Very coarse materials like boulders and blocks along with some minor fractions of
rock debris carried into this stream settle in the valley of ice beneath the glacier and
after the ice melts can be found as a sinuous ridge called esker.

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 Outwash Plains

• The plains at the foot of the glacial mountains or beyond the limits of continental ice
sheets are covered with glacio-fluvial deposits in the form of broad flat alluvial fans
which may join to form outwash plains of gravel, silt, sand and clay.

Drumlins

• Drumlins are smooth oval shaped ridge-like features composed mainly of glacial till
with some masses of gravel and sand.

• The long axes of drumlins are parallel to the direction of ice movement.

• They may measure up to 1 km in length and 30 m or so in height.

• One end of the drumlins facing the glacier called the stoss end is blunter and steeper
than the other end called tail.

• The drumlins form due to dumping of rock debris beneath heavily loaded ice
through fissures in the glacier. The stoss end gets blunted due to pushing by moving
ice.

• Drumlins give an indication of direction of glacier movement.

Waves and Currents

• When waves break, the water is thrown with great force onto the shore, and
simultaneously, there is a great churning of sediments on the sea bottom.

• Constant impact of breaking waves drastically affects the coasts. Storm waves and
tsunami waves can cause far-reaching changes in a short period of time than normal
breaking waves. As wave environment changes, the intensity of the force of breaking
waves changes.

• Other than the action of waves, the coastal landforms depend upon:
(i) the configuration of land and sea floor;
(ii) whether the coast is advancing (emerging) seaward or retreating (submerging)

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 landward.

• Assuming sea level to be constant, two types of coasts are considered to explain the
concept of evolution of coastal landforms:
(i) high, rocky coasts (submerged coasts);
(ii) low, smooth and gently sloping sedimentary coasts (emerged coasts).

High Rocky Coasts

• Along the high rocky coasts, the rivers appear to have been drowned with highly
irregular coastline.
The coastline appears highly indented with extension of water into the land where
glacial valleys (fjords) are present.

• The hill sides drop off sharply into the water. Shores do not show any depositional
landforms initially.

• Erosion features dominate along high rocky coasts, waves break with great force
against the land shaping the hill sides into cliffs.

• With constant pounding by waves, the cliffs recede leaving a wave-cut platform in
front of the sea cliff. Waves gradually minimise the irregularities along the shore.

• Bars are submerged features and when bars show up above water, they are called
barrier bars.
Barrier bar which get keyed up to the headland of a bay is called a spit.

• When barrier bars and spits form at the mouth of a bay and block it, a lagoon forms.
The lagoons would gradually get filled up by sediments from the land giving rise to a
coastal plain.

Low Sedimentary Coasts

• Along low sedimentary coasts the rivers appear to extend their length by building
coastal plains and deltas. The coastline appears smooth with occasional incursions of
water in the form of lagoons and tidal creeks.

• The land slopes gently into the water. Marshes and swamps may abound along the

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 coasts.

• When waves break over a gently slopingsedimentary coast, the bottom sediments
get churned and move readily building bars, barrier bars, spits and lagoons.

• Lagoons would eventually turn into a swamp which would subsequently turn into a
coastal plain.

• The west coast of our country is a high rocky retreating coast. Erosional forms
dominate in the west coast.

• The east coast of India is a low sedimentary coast. Depositional forms dominate in
the east coast.

Eroisonal Landforms

Cliffs, Terraces, Caves and Stacks

• Wave-cut cliffs and terraces are two forms usually found where erosion is the
dominant shore process.

• Almost all sea cliffs are steep and may range from a few metre to 30 metre or even
more. At the foot of such cliffs there may be a flat or gently sloping platform covered
by rock debris derived from the sea cliff behind.

• Such platforms occurring at elevations above the average height of waves is called a
wave-cut terrace.

• The lashing of waves against the base of the cliff and the rock debris that gets
smashed against the cliff along with lashing waves create hollows and these hollows
get widened and deepened to form sea caves.

• The roofs of caves collapse and the sea cliffs recede further inland. Retreat of the cliff
may leave some remnants of rock standing isolated as small islands just off the shore.
Such resistant masses of rock, originally parts of a cliff or hill are called sea stacks.

Depositional Landforms

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 Beaches and Dunes

• Beaches are characteristic of shorelines that are dominated by deposition, but may
occur as patches along even the rugged shores.

• Most of the sediment making up the beaches comes from land carried by the
streams and rivers or from wave erosion. Beaches are temporary features.

• Most of the beaches are made up of sand sized materials. Beaches called shingle
beaches contain excessively small pebbles and even cobbles.

• Just behind the beach, the sands lifted and winnowed from over the beach surfaces
will be deposited as sand dunes. Sand dunes forming long ridges parallel to the
coastline are very common along low sedimentary coasts.

Bars, Barriers and Spits

• A ridge of sand and shingle formed in the sea in the off-shore zone (from the
position of low tide waterline to seaward) lying approximately parallel to the coast is
called an off-shore bar.

• An off-shore bar which is exposed due to further addition of sand is termed a barrier
bar.

• The off-shore bars and barriers commonly form across the mouth of a river or at the
entrance of a bay. Sometimes such barrier bars get keyed up to one end of the bay
when they are called spits. Spits may also develop attached to headlands/hills. The
barriers, bars and spits at the mouth of the bay gradually extend leaving only a small
opening of the bay into the sea and the bay will eventually develop into a lagoon.

• The lagoons get filled up gradually by sediment coming from the land or from the
beach itself (aided by wind) and a broad and wide coastal plain may develop replacing
a lagoon.

• The coastal off-shore bars offer the first buffer or defence against storm or tsunami
by absorbing most of their destructive force. Then come the barriers, beaches, beach
dunes and mangroves, if any, to absorb the destructive force of storm and tsunami
waves. So, if we do anything which disturbs the ‘sediment budget’ and the mangroves

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 along the coast, these coastal forms will get eroded away leaving human habitations
to bear first strike of storm and tsunami waves.

Winds

• Wind is one of the two dominant agents in hot deserts. The desert floors get heated
up too much and too quickly because of being dry and barren.

• The heated floors heat up the air directly above them and result in upward
movements in the hot lighter air with turbulence, and any obstructions in its path sets
up eddies, whirlwinds, updrafts and downdrafts.

• Winds also move along the desert floors with great speed and the obstructions in
their path create turbulence. Of course, there are storm winds which are very
destructive.

• Winds cause deflation, abrasion and impact.

• Deflation includes lifting and removal of dust and smaller particles from the surface
of rocks. In the transportation process sand and silt act as effective tools to abrade
the land surface.

• The impact is simply sheer force of momentum which occurs when sand is blown
into or against a rock surface. It is similar to sand- blasting operation.

• The desert rocks devoid of vegetation, exposed to mechanical and chemical

weathering processes due to drastic diurnal temperature changes, decay faster and
the torrential rains help in removing the weathered materials easily.

• That means, the weathered debris in deserts is moved by not only wind but also by
rain/sheet wash.
The wind moves fine materials and general mass erosion is accomplished mainly
through sheet floods or sheet wash. Stream channels in desert areas are broad,
smooth and indefinite and flow for a brief time after rains.

Erosional Landforms

Pediments and Pediplains

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 • Gently inclined rocky floors close to the mountains at their foot with or without a
thin cover of debris, are called pediments.

• Such rocky floors form through the erosion of mountain front through a
combination of lateral erosion by streams and sheet flooding.

• Once, pediments are formed with a steep wash slope followed by cliff or free face
above it, thesteep wash slope and free face retreat backwards.

• This method of erosion is termed as parallel retreat of slopes through back wasting.
So,
through parallel retreat of slopes, the pediments extend backwards at the expense of
mountain front, and gradually, the mountain gets reduced leaving an inselberg which
is a remnant of the mountain.

• That’s how the high relief in desert areas is reduced to low featureless plains called
pediplains.

Playas

• Plains are by far the most prominent landforms in the deserts. In basins with
mountains and hills around and along, the drainage is towards the centre of the basin
and due to gradual deposition of sediment from basin margins, a nearly level plain
forms at the centre of the basin.

• In times of sufficient water, this plain is covered up by a shallow water body. Such
types of shallow lakes are called as playas where water is retained only for short
duration due to evaporation and quite often the playas contain good deposition of
salts. The playa plain covered up by salts is called alkali flats.

Deflation Hollows and Caves

• Weathered mantle from over the rocks or bare soil, gets blown out by persistent
movement of wind currents in one direction. This process may create shallow
depressions called deflation hollows.

• Deflation also creates numerous small pits or cavities over rock surfaces. The rock

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 faces suffer impact and abrasion of wind-borne sand and first shallow depressions
called blow outs are created, and some of the blow outs become deeper and wider fit
to be called caves.

Mushroom, Table and Pedestal Rocks

• Many rock-outcrops in the deserts easily susceptible to wind deflation and abrasion
are worn out quickly leaving some remnants of resistant rocks polished beautifully in
the shape of mushroom with a slender stalk and a broad and rounded pear shaped cap
above.

• Sometimes, the top surface is broad like a table top and quite often, the remnants
stand out like pedestals.

Depositional Landforms

• Wind is a good sorting agent. Depending upon the velocity of wind, different sizes of
grains are moved along the floors by rolling or saltation and carried in suspension and
in this process of transportation itself, the materials get sorted.

• When the wind slows or begins to die down, depending upon sizes of grains and
their critical velocities, the grains will begin to settle. So, in depositional landforms
made by wind, good sorting of grains can be found.

• Wind is there everywhere and wherever there is good source of sand and with
constant wind directions, depositional features in arid regions can develop anywhere.

Sand Dunes

• Dry hot deserts are good places for sand dune formation. Obstacles to initiate dune
formation are equally important. There can be a great variety of dune forms.

• Crescent shaped dunes called barchans with the points or wings directed away from
wind direction i.e., downwind, form where the wind direction is constant and
moderate and where the original surface over which sand is moving is almost uniform.

• Parabolic dunes form when sandy surfaces are partially covered with vegetation.
That means parabolic dunes are reversed barchans with wind direction being the

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 same.

• Seif is similar to barchan with a small difference. Seif has only one wing or point. This
happens when there is shift in wind conditions. The lone wings of seifs can grow very
long and high.

• Longitudinal dunes form when supply of sand is poor and wind direction is constant.
They appear as long ridges of considerable length but low in height.

• Transverse dunes are aligned perpendicular to wind direction. These dunes form
when the wind direction is constant and the source of sand is an elongated feature at
right angles to the wind direction. They may be very long and low in height.

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